Composite structures comprising parts that have different coefficients of thermal expansion may induce residual stresses in the structure during thermal curing or other fabrication processes that cause the parts to thermally expand at different rates. For example, and without limitation, hybrid composite structures comprising a metal reinforced with carbon fiber polymers may be subject to thermal induced distortion while curing at elevated temperatures. In some cases, this problem may be addressed by using fiber reinforced polymers that cure at room temperature, or which may be cured using various forms of radiation. However, room temperature cure polymers may have a short working life, long cure times, and require extra clean-up work. Room temperature cure polymers may also exhibit reduced performance characteristics compared to polymers that are cured at elevated temperatures. Known radiation cured adhesives may be more brittle than desired and exhibit lower than desired toughness, which may render these adhesives unsuitable for some applications, particularly where resistance to impact loads is valued.
Interpenetrating polymer networks (IPN) have been used as an adhesive in composite structures. IPNs are based on polymer systems that cure at different temperatures using differing cure mechanisms, but may exhibit properties that are superior to those of their constituent polymer systems. However, IPNs have not been adapted for solving the problem of residual stresses that are induced in composites by the differential expansion of the components of which the composite is formed.
Accordingly, there is a need for an IPN adhesive that may be used to reduce or eliminate residual stresses in composite structures that may be caused by differential expansion of differing components used in the structure, such as metals reinforced with carbon fiber polymers. There is also a need for a method of making composite structures that employ IPN adhesives to reduce or eliminate the residual stresses.